The invention relates to electrolytic capacitors and the use of separator papers therein.
The xe2x80x9cworkingxe2x80x9d or xe2x80x9cfillxe2x80x9d electrolyte solutions contained within aluminum electrolytic capacitors generally contain a small amount of water due to contact with the atmosphere and to absorption of water from the separator papers used to prevent contact between anode and cathode foils. Electrolyte solutions for use in higher voltage aluminum electrolytic capacitors may contain 2 to 5% or more of water intentionally added to the electrolyte for the purpose of facilitating the repair of the anodic oxide film on the anode foil and for the purpose of reducing the electrolyte resistivity.
The water present in the xe2x80x9cfillxe2x80x9d electrolyte may react with the cathode foil at elevated temperatures, both during use and while the devices are standing idle. The water present in the electrolyte may also attack the anode foil while the devices stand idle (i.e., un-electrified) at elevated temperature. The reaction with water contained in the working electrolyte produces a bulky reaction product on the cathode foil, which reduces device capacitance, increases device ESR and dissipation factor, and may rupture the seals of the devices due to the liberation of hydrogen gas by the reaction. The reaction of the water contained in the working electrolyte with the anodic oxide (while standing un-electrified at elevated temperature) increases the leakage current and dissipation factor of the devices.
For many years relatively small amounts (i.e., 50 ppm to 0.5%) of soluble orthophosphate salts have been included in working electrolyte formulations for aluminum electrolytic capacitors for the purpose of maintaining a passive, hydration resistant surface on both the anode and cathode foils of these devices. The orthophosphate absorbs onto and reacts with the surfaces of the anode and cathode foils, rendering these surfaces much less reactive with the moisture contained in the working electrolyte.
Orthophosphate contained in the working electrolyte in aluminum electrolytic capacitors tends to concentrate adjacent to the anodic oxide surfaces during electrification of the device. At higher operating voltages the concentration of orthophosphate may occur to a sufficient extent to significantly reduce the pH of the solution adjacent to the anodic oxide. If the concentration of phosphate is sufficient high, the oxide electrical resistance is compromised (probably due to the low pH and local oxide dissolution) and the devices fail due to short-circuiting of the anodic oxide dielectric. The occurrence of device short-circuiting due to the presence of excessive amounts of phosphate adjacent to the anodic oxide during device electrification has led capacitor manufacturers to include very small levels of phosphate in electrolytes for higher operating voltage capacitors or even to eliminate orthophosphate altogether from high voltage working electrolytes. The reduced level or elimination of phosphate from high voltage working electrolytes tends to render the devices susceptible to the hydration attack due to the water content of the electrolyte, as described above.
The invention is directed to a separator paper comprising paper impregnated with soluble silicate. The soluble silicate may be applied to the separator paper by immersing the paper into an aqueous solution of the soluble silicate or by spraying with an aqueous solution of the soluble silicate. In a preferred embodiment, the soluble silicate is sodium silicate.
The invention is further directed to an electrolytic capacitor comprising an electrolytic solution, at least one anode foil, at least one cathode foil, and separator paper between the at least one anode foil and at least one cathode foil, wherein the separator paper comprises paper impregnated with a soluble silicate.
In a preferred embodiment, the at least one anode foil and at least one cathode foil are aluminum foil. The soluble silicate leaches from the paper to provide about 10 to about 5000 ppm silicate in the electrolytic solution.